The present invention relates to a heat treatment boat for heat treating disk-shaped objects such as semiconductor wafers.
Semiconductor wafer (hereinafter called "wafer") preparation includes, as preparation steps heat treatments at high temperatures for formation of oxide films, diffusion of dopants, etc. As apparatuses for such heat treatments, horizontal heat treatment furnaces have been dominant, but recently vertical heat treatment furnaces are increasingly used because they draw in less outside atmosphere.
A vertical heat treatment apparatus using a vertical heat treatment furnace uses a vertical heat treatment boat (also called "wafer boat") for mounting a number of wafers at a set vertical interval to load/unload the wafers into the heat treatment furnace.
FIG. 18 shows a conventional heat treatment wafer boat. The conventional heat treatment wafer boat 1 includes four support rods of 13-16 of, e.g., quartz erected between a disk-shaped top plate 11 and a disk-shaped bottom plate 12. Two (13, 14) of the support rods respectively support the wafers at the front left and right in the direction of advance of the wafers W as viewed in FIG. 18, and the other two (15, 16) of the support rods support the wafers at the rear left and right in the direction of advance of the wafers as viewed in FIG. 18. The wafer boat 1 is provided on a heat insulation cylinder 2 which is formed of a heat insulating material.
As shown in FIG. 19, each support rod 13-16 has grooves 17 with a vertical width which is a little larger than a thickness of the wafers W so that the respective grooves support the respective wafers W at the undersides of the peripheral portions. The wafers W are put in/out of the grooves 17 by a transfer arm 21 in the direction of the arrow between the two front support rods 13, 14.
Such structure of the heat treatment boat 1 has been used in conventional horizontal heat treatment furnaces and is used as it is in the vertical heat treatment furnaces. That is, in the horizontal heat treatment furnace, the wafers are transferred by a push-up mechanism for pushing up the wafers from below the heat treatment boat and a mechanism for holding the lifted wafers W. Such transfer has determined the structure of the heat treatment boat.
In the heat treatment boat 1 of FIG. 18, when a set number of wafers W to be treated are mounted on the boat 1, an elevator 22 is lifted to load the boat into the heat treatment furnace, so that the wafers W are loaded in the heat treatment furnace to be subjected to a required heat treatment.
In a heat treatment for processing wafers, e.g., to diffuse dopant ions implanted in the wafers down to a set depth, the wafers are heated at a high temperature of about 1200.degree. C. for a long period of time. In this case, the substrate material of the wafers is silicon, since the melting point of silicon is 1410.degree. C., the silicon wafers have an extremely low yield stress at 1200.degree. C.
On the other hand, there has been a tendency toward increased wafer diameter. Their size has increased from 6 inches to 8 inches, and further 12 inch-diameter wafers are being studied. In wafers of such large diameters, when the wafers are heat treated at a temperature near a melting point of their substrate material, crystal deformations called slip tend to occur near portions of the wafers at which the wafers are supported by the support rods of the heat treatment boat. These slips are fine faults which are invisible but can be seen by magnifying glasses, microscopes. etc.
As shown in FIG. 18, the top and the bottom plates 11, 12 of the wafer boat 1 are heated by a heater in the furnace, and because of no heaters above and below the wafer boat 1 heat is radiated as secondary radiant heat from the top and the bottom plates 11, 12. Since the top and the bottom plates 11, 12 are plane bodies, amounts of the radiant heat are considerably large. Accordingly heat is conducted from the support rods 11-14 of the wafer boat to the top and the bottom plates 11, 12, i.e., from the wafers W to the support rods to the top and the bottom plates 11, 12, temperature gradients occur in the peripheral portions of the wafers W.
To consider the occurrences of the slips, the micro-areas of the wafers in actual contact with the grooves 17 as shown in FIG. 20 are noted. Large stresses will be applied to these micro-areas as described above, and large temperature differences will take place there, so that large thermal stresses will be applied thereto, and planes of the wafers W will be displaced past each other from the bottom to the top of the wafers W, causing the slips. It is considered that larger calories flow especially from those of the wafers nearer to the top and the bottom ends of the wafer boat 1 to the top and the bottom ends thereof, so that the slips tend to occur in those wafers W. When the slips occur in the regions of the wafers W for the devices to be formed in, the devices will be defective with low yields.
Causes for the slips will be 1) internal stresses due to an own weight of the wafers W, and 2) thermal strain stresses due to disuniform temperatures in plane of the wafers W. In connection with the cause 1), the support positions by the heat treatment boat 1 are located on the peripheral parts of the wafers and partially at four positions, so that large internal stresses will occur due to an own weight of the wafers near the supported parts, and the slips will occur when the internal stresses exceed a certain magnitude. In addition, wafers have soris(bendings) not only within the allowed range, but also due to temperature distributions when heated. Furthermore, a width of the grooves in the support rods have manufacturing errors, If the wafers should be apart from any one of the four support positions because of any of these causes, the wafers would be supported only at three of the four support positions, so that, as seen from the layout of the support rods 13-16, a load at the respective support positions will become imbalanced, with the result a large stress exceeding a threshold for the occurrence of the slips will take place at one of the four support positions.
As regards the cause 2) above, when the wafers are heated, heat is supplied and removed through the support rods of the heat treatment, so that temperature differences take place between the central parts of the wafers and the peripheral parts thereof, and thermal strain stresses occur. The slips will occur when the thermal strain stresses exceed a certain magnitude.
Thus, in heat treating wafers, especially in heat treating wafers at a high temperature near a melting point of the substrate of the wafers, as the wafers have larger diameters, the problem of the slips take place. This problem is a serious obstacle to increasing diameters of wafers.
A further cause which is considered for causing the slips is thermal strain stresses due to disuniform temperatures in plane of wafers. With reference to FIG. 21, heat is supplied and removed through support rods 3A of a wafer boat 3 when wafers W are heated. Accordingly temperature differences take place between the central parts of the wafers and the peripheral parts thereof, with a result of occurrence of thermal strain stresses. The slips will occur when the thermal strain stresses exceed a certain magnitude. The slips tend to occur especially in lower part of the wafer boat 3.
The reason for this will be as follows. In a case that a heat treating temperature is above, e.g., 1000.degree. C., the wafer boat 3 is loaded into a reaction tube 1 which has been pre-heated up to about 800.degree. C., and when the wafers W are heated up to an atmospheric temperature, the temperature in the reaction tube 1 is raised up to a set heat treating temperature. Quartz forming the reaction tube 1 has the low heat conductivity and has good properties as a heat insulating material. On the other hand, because of its high light transmittance, quartz does not much absorb radiant heat and is slow to be heated. Accordingly a heat insulator 2 is behind the wafer boat to be heated until the wafer boat 3 is loaded in the reaction vessel 1 and has a stabilized atmospheric temperature and until the interior of the reaction tube 1 is heated up to a set temperature, so that heat escapes due to a temperature difference between the two from the lower part of the wafer boat 3 to the heat insulator 2, and a temperature of the lower part of the wafer boat 3 cannot be easily raised. As a result, temperature differences occur between parts of the wafers W supported by the wafer boat 3, and the rest parts of the wafers W, and the slip will easily take place.